High rate transmission (e.g., 112 Gbps) of data serially over one physical communication channel (link) may be difficult to implement. In order to allow the transmission over high data rate communication links, current transmission standards define the use of Multi Lane Distribution (MLD), according to which the data is distributed and transmitted over several virtual lanes, each of which is sent over a physical layer at a lower data rate. The MLD provides flexibility in the implementation of physical layer lanes.
A Forward Error Correction (FEC) mechanism is normally used at the receiver side, in order to accurately reconstruct the transmitted data. The FEC enables relaxing SNR (and BER) requirements from the physical layer link.
As an example of FEC mechanism, the standard “Generic Forward Error Correction” GFEC is a Reed-Solomon coding (a non-binary cyclic error-correcting code) that allows certain amount of errors to be acceptable in the physical layer lanes, thus, a transmission with low Signal to Noise Ratio (SNR) is enabled.
The Optical Transport Lane OTL4.4 MLD (G.709) standard defines the method to map Optical Transport Unit 4 (OTU4) over 4 optical lanes. The basic Optical Transport Network (OTN—refers to networks using the ITU-T Recommendation G.709 “Interfaces for the Optical Transport Network (OTN)” describes a means of communicating data over an optical network standard for Wavelength Division Multiplexed signals) frame structure (that includes FEC) remains unchanged and frames are serialized into a stream of blocks. These blocks are then distributed in a round-robin manner (an arrangement of choosing all elements in a group equally in some specified order, usually from the top to the bottom of a list and then starting again at the top of the list and so on) over 20 virtual lanes (VLs). The VLs are usually transmitted over 4 optical lanes (each optical lane incorporates 5 VLs). At the receiver side, the VLs are recovered, reordered and de-skewed in order to recreate the OTN frame. Thus, any lane skew generated in the physical layer is compensated.
GFEC, standard Reed Salomon 239/255, fails to correct codewords and displays post FEC errors in case more than 8 bytes in a 255 byte codeword contain errors. In case the link errors are not uniformly distributed, i.e. there are correlated consecutive errors, some codewords contain significantly more errors than others, and the probability for post FEC errors is increased. This is the case when using both GFEC and OTL4.4 MLD. FIG. 1 (prior art) illustrates the structure of an OTN frame, according to the G.709 standard. Logically, each OTN frame consists of 4 rows of 4080 bytes each. Since the bytes cannot be transmitted serially (due to the very high data rate), the standard suggests using MLD.
FIG. 2 (prior art) illustrates the implementation of data transmission using 20 virtual lanes, where the byte-stream of a frame is divided into 16-byte (128 bit) segments. The physical layer is divided into 4 physical lanes, where each physical lane is used to carry 5 VLs. The segments are distributed round-robin to 20 VLs. There are 255 segments in each row and 1020 segments in each frame. This allows the physical layer to operate at much lower data rate (in this example, 28 Gbps). In this case, the physical layer transmits one bit from each VL with interleaving (a process for arranging the data in a noncontiguous manner).
However, using the G.709 transmission standard creates a problem of interaction between the Multi Lane Distribution (MLD) and the Generic Forward Error Correction (GFEC) mechanism at the receiver side. This interaction leads to degradation in the channel's performance, when the reception errors induces by the channel's distortion are correlated errors (i.e., that appear in bursts, rather than being uniformly spread). In this case, the ability of the GFEC is limited, and the channel becomes much more vulnerable.
Some solutions to this interaction problem use nonstandard error correction mechanisms which eliminate this interaction, but this requires proprietary, nonstandard transmission format, requiring full control of both the transmitting and receiving sides. The GFEC can correct only up to 8 bytes erroneous in a codeword (is a certain binary sequence from a dictionary of allowable words. After transmission over a noisy channel, it is possible to check if the received binary sequence is in the dictionary of codewords and if not, choose the codeword that is most similar to what was received). When a byte in a codeword contains an error in one bit (or more), the entire byte is considered erroneous by the GFEC mechanism. Even if there are 8 erroneous bits in the same byte, still the GFEC mechanism can correct it. However, in case when the 8 erroneous bits are distributed among different bytes, the GFEC mechanism will not be able to correct the errors.
FIG. 3 (prior art) illustrates the distribution of VLs within an OTN frame, according to the G.709 transmission standard. In this example, the first bit transmitted over the channel will be mapped to the first byte of VL0 (red rectangle), the second bit over the channel will be mapped to the first byte of VL1 (red rectangle) and so forth. It can be seen that even though these bits were transmitted over the channel in a consecutive manner, in the OTN frame at the receiver they will be spaced apart by an entire segment. Here, the first byte of each segment has been mapped into the same codeword of the GFEC mechanism.
FIG. 4a (prior art) illustrates the ordering of the VLs with a burst of 5 consecutive erroneous bytes (correlative errors which are marked as black rectangles) which were mapped as the second byte in each segment.
FIG. 4b (prior art) illustrates the ordering of the GFEC codeword with mapping of this burst of 5 consecutive erroneous bytes (marked as black rectangles) into the same FEC codeword. Here, a single line side lane of 28 Gbps contains 5 VLs. The VLs are ordered in rows by lane segments of 16 bytes each. The GFEC codeword is made of one byte of each segment. Hence, each GEFC codeword contains one byte from each VL. Thus, in case there are adjacent errors in the 28 Gbps line side they are expected to be mapped into different bytes of the same codeword. When using GFEC over OTL4.4 MLD, adjacent errors generated in the line side will be mapped into different bytes of one GFEC codeword, thereby causing a higher probability for post FEC errors. It can be seen that due to the interaction between the MLD and the GFEC mechanism at the receiver side, all 5 erroneous bytes were mapped to the same codeword (Codeword 1 in this example) of the GFEC mechanism. This concentration of erroneous bytes in the same codeword eliminates the capability of the GFEC mechanism to correct them.
It is therefore an object of the present invention to provide a method for increasing the probability of error correction in an optical communication channel, which complies with the G.709 transmission standard.
It is another object of the present invention to provide a method for increasing the probability of error correction in an optical communication channel, which reduces vulnerability of the optical communication channel to noise and correlated errors.
Other objects and advantages of the invention will become apparent as the description proceeds.